Study finds agricultural land use may help pest fish

A new study shows that degradation of freshwater streams is helping a pest fish increase in abundance and spread more widely into waterways, according to scientists at the University of Auckland.

School of Environment researchers including doctoral candidate Finn Lee, Associate Professor Kevin Simon, and Professor George Perry, studied 31 streams in the North Island looking at agricultural land use alongside the abundance and spread of mosquitofish (Gambusia affinis).

Once thought to be an effective mosquito control agent, Mosquitofish were widely introduced into waterways in the early 20th Century, including in the United States, Europe and Australia. Aggressive and predatory, they have been responsible for the decline of native fish and frog species in some countries while having little effect on mosquito populations in some places, but mosquitofish are poorly studied in New Zealand.

The latest study found high abundance of mosquitofish when conditions usually associated with agricultural land use were also present, in particular low water flow, higher nutrient levels and higher water temperature.

Abundant mosquitofish populations were also found where higher levels of aquatic plants called macrophytes were present. Macrophytes are also associated with more intensive land use.

“Overall our findings suggest that in addition to reducing water quality, agricultural land use may be improving conditions for these invasive fish by altering the streams to suit them,” says Mr Lee who was lead author on the study.

“Habitat conditions we commonly associate with agricultural land use make it easier for mosquito fish to invade waterways and thrive, so water degradation is a key factor in helping mosquitofish spread.”

The study findings will help scientists investigate land management techniques that could better control the numbers and spread of mosquitofish.

“Now that we are getting a better idea of how agricultural land use and mosquitofish populations might be linked, we can look at land management techniques that will improve water and habitat quality to better control mosquitofish populations,” Associate Professor Simon says.

Dr Simon has funding through the Marsden Fund for further study of populations of mosquitofish both here and in the United States.